Conventionally, continuous charging and pulse charging are common methods for charging a nonaqueous second battery such as a lithium ion battery, an Ni—MH battery, etc. Continuous charging may be grouped into CC (constant current) charging, CV (constant voltage) charging, and CCCV (constant-current constant-voltage) charging, which combines the CC charging and the CV charging. In the charging of a nonaqueous second battery, it is necessary to exercise due care to avoid an excess charging voltage exceeding a predetermined voltage since such an excessive charging voltage considerably deteriorates the battery performance.
Thus, in order to avoid excessive charging voltage, the CCCV charging is generally used as the continuous charging method. In the CCCV charging method, charging with a constant current is performed at an early stage of the charging to the second battery and charging with a constant voltage is subsequently performed after a battery voltage of the second battery reaches a predetermined voltage. When the charging current (also referred to herein as “charge current” and “battery current”) is reduced to a predetermined value, the second battery is in a fully-charged state and the charging is completed. One advantage of this method is the possibility of rapid charging by setting the charging current used in the constant current charging during the early stage of the charging to a relatively large value. Another advantage is the prevention of deterioration of the second battery due to application of excessive voltage since the charging mode is shifted from CC charging to CV charging when the battery voltage of the second battery reaches a predetermined voltage.
However, when the constant current used in the initial charging stage is set to an excessively large value, a heat production by the second battery becomes large, which causes several problems: a reduction of charging efficiency, an acceleration of deterioration with respect to the second battery, etc. On the other hand, pulse charging has an advantage of less deterioration to the second battery because pulse charging is provided with pause times at certain intervals during the charging period, which increases efficiency of electrochemical changes. Additionally, pulse charging allows a relatively large charging current and therefore, is suitable for rapid charging as well.
For example, Japanese Laid-Open Patent Application Publication No. 2001-169471 describes a second battery charging apparatus that features advantages of both continuous charging and pulse charging. This apparatus attempts to avoid over-charging while suppressing heat production of the second battery. That is, the continuous charging is performed during the initial charging and the charging mode is switched to the pulse charging when the battery voltage in the charging exceeds a predetermined voltage V1 depending on battery temperature. When the battery voltage with an open circuit in pulse charging exceeds a predetermined voltage V3, the mode is switched from the pulse charging to the continuous charging. Then, the charging is stopped when the battery voltage in the second battery reaches a predetermined voltage V2.
For such a charging apparatus, miniaturization has increasingly been an issue recently, as mobile equipment such as cellular phones and the like using a second battery becomes widespread. In particular, charging apparatus for cellular phones are often carried along with a cellular phone. Such mobility requires a relatively short charging time to make the cellular phone operable and also requires a compact and light body producing less heat. The charge required to make the cellular phone operable may not be a full charge. The charge time to reach less than fully-charged state may be correspondingly less than the time to fully charge the second battery.
However, conventional charging apparatus have been designed with a view mainly to reducing a charging time to the full-charge state and/or suppressing a temperature rise of the battery. To reduce the charging time, it is necessary to charge with a relatively large current. As a result, the charging apparatus produces a relatively large amount of heat and accordingly the size of the charging apparatus may become large. Since the temperature of the second battery rises, a temperature detection mechanism is additionally needed, resulting in an increase of the size and complexity of the apparatus itself. In addition, charging with such large current often accelerates deterioration of ionization with respect to the second battery.